Biodiesel which is produced from the oil of oily plants such as rape seed, soybean and palm is typical biodiesel which has already been commercialized, and the production thereof is increasing rapidly worldwide. However, because raw material crops for producing biodiesel are expensive, biodiesel is disadvantageous in terms of production costs and less competitive compared to crude oil-derived diesel. Thus, despite its various advantages in terms of environment and agricultural economy, biodiesel appears to have no commercial competitiveness, if there is no tax reduction. The increase in raw oil prices by energy exhaustion is expected to give commercial competitiveness to biodiesel, but a sudden rise in the price of raw crops by a recent increase in the production of biodiesel becomes a new factor that reduces the competitiveness of biodiesel.
Moreover, photosynthetic oil from oily plants and photosynthetic microalgae, which is the major source of bio-oil for producing biodiesel, has important advantages in that it uses sunlight and recycles carbon dioxide, but it is disadvantageously influenced by various factors, including time, space, seasons and climate. Also, an increase in the use of biodiesel which is produced from photosynthetic oil can cause food shortages and new environmental problems by the mass cultivation of raw material crops, and thus there are doubts on the effectiveness of biodiesel from photosynthetic oil.
For these reasons, the fermentation of organotrophic microorganisms has received attention as a method for the mass production of bio-oil. Typical microorganisms that produce oil include Chlorella protothecoides, Yarrowia lipolytica, Rhodosporidium toruloides, Rhodotorula glutinis and the like, and studies on processes for the fermentation thereof have been actively conducted.
Among these oily microorganisms, microalgae belonging to the thraustochytrid family are oily microorganisms capable of producing bio-oil, which contains polyunsaturated fatty acids such as DHA (docosahexaenoic acid), in an amount of up to 70% of the dry cell weight. DHA, a fatty acid essential for brains, ocular tissue and nervous systems, is known to play an important role in the sight of infants and the development of motor nerves. In addition, it was reported that the amount of DHA is significantly reduced in the brain of dementia patients, and DHA has been reported to have various new functions, such as inhibition of age-related macular degeneration. Despite such useful physiological functions, the human body cannot self-synthesize a sufficient amount of DHA. Thus, DHA is recognized as an essential nutrient to be supplied from the outside, many global organizations, including the World Health Organization, recommend that 1 g or more per day of DHA be taken. Thus, DHA is being commercialized as various products, including health functional foods, and the utility thereof as a raw material for medical drugs is high, indicating that DHA has a very high commercial value. Thus, oil from the fermentation of microalgae belonging to the thraustochytrid family can provide a connection between high-value-added industries utilizing DHA and the biodiesel industry, thereby providing commercial competitiveness to biodiesel, unlike general microbial oil or photosynthetic oil.
However, the most important factor for ensuring the commercial competitiveness of microbial fermentation oil as a raw material for biodiesel is the use of industrial waste, waste resources and surplus biomass as nutrient sources, and ultimately the use of abundant non-edible cellulosic biomass. Examples of non-food cellulosic biomass resources include woody biomass, agricultural byproducts, urban waste and so on.
A number of methods of producing DHA (docosahexaenoic acid) using microorganisms of the thraustochytrid family have been reported. Such methods are mainly methods of producing DNA by culturing microorganisms of the thraustochytrid family in media containing glucose as a carbon source (Korean Patent Laid-Open Publication No. 2008-0087820, Korean Patent Laid-Open Publication No 2009-0064603, US Patent Publication No. 20080009045, and US Patent Publication No. 20050019880). However, there has been no report on a method of producing bio-oil using non-food cellulosic biomass that is the next-generation biomass resources.
Accordingly, the present inventors have made extensive efforts to develop a method of producing DHA in a high yield using microalgae and, as a result, have found that novel thraustochytrid-like microalgae isolated from soil in a mangrove area in Malaysia contain a high concentration of DHA and that bio-oil containing DHA is produced when the novel microalgae are cultured using bean powder or cellulosic biomass as a nutrient source, thereby completing the present invention.